1. Field of the Invention
The present invention relates generally to the field of fuel cells and, more specifically, to a membrane for controlling water and fuel carryover in a direct oxidation fuel cell.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive choices for fuel due to the their high specific energy.
Fuel systems may be divided into “reformer-based” (i.e., those in which the fuel is processed in some fashion before it is introduced into the cell) or “direct oxidation” in which the fuel is fed directly into the cell without internal processing. Most currently available fuel cells are of the reformer-based type, but fuel-processing requirements for such cells limits the applicability of those cells to relatively large systems.
Direct oxidation fuel cell systems may be better suited for a number of applications such as smaller mobile devices (i.e., mobile phones, handheld and laptop computers), as well as in larger applications. One example of a direct oxidation system is the direct methanol fuel cell system or DMFC. In a DMFC, the electrochemical reaction at the anode is a conversion of methanol and water to CO2, H+ and e−. More specifically, a liquid carbonaceous solution (typically aqueous methanol) is applied to a protonically-conductive (but, electronically non-conductive) membrane (PCM) directly using a catalyst on the membrane surface to enable direct oxidation of the hydrocarbon on the anode. The hydrogen protons are separated from the electrons and the protons pass through the PCM, which is impermeable to the electrons. The electrons thus seek a different path to reunite with the protons and travel through a load, providing electrical power.
The carbon dioxide, which is essentially a waste product, is separated from the remaining methanol fuel mixture before such fuel is re-circulated. In an alternative usage of the carbon dioxide this gas can be used to passively pump liquid methanol into the feed fuel cell. This is disclosed in U.S. patent application Ser. No. 09/717,754, filed on Dec. 8, 2000, for a PASSIVELY PUMPED LIQUID FEED FUEL CELL SYSTEM, which is commonly owned by the assignee of the present invention, and which is incorporated by reference herein in its entirety.
Fuel cells have been the subject of intensified recent development because of their high energy density in generating electric power from fuels. This has many benefits in terms of both operating costs and environmental concerns. Adaptation of such cells to mobile uses, however, is not straightforward because of technical difficulties associated with reforming carbonaceous fuels in a simple and cost effective manner, and within acceptable form factors and volume limits. Further, a safe and efficient storage means for hydrogen fuel gas (or hydrogen fuel gas reformats) a challenge because hydrogen gas must be stored at high pressure and at cryogenic temperatures or in heavy absorption matrices in order to achieve useful energy densities. It has been found, however, that a compact means for storing hydrogen is in a hydrogen rich compound with relatively weak chemical bonds, such as methanol (and to a lesser extent, ethanol, propane, butane and other carbonaceous liquids). Thus, efforts to develop the DMFC commercially have increased over the past several years.
Even in a DMFC, however, the stored reactants may constitute a significant portion of the total of the volume of the system, thus creating a need to store the fuel in undiluted form even though an aqueous solution may be preferable when the fuel is actually presented to the membrane for reaction. For example, some current generation DMFCs operate on an aqueous solution that is 3% methanol. This implies that water must be added to the undiluted fuel between the fuel source and the reactive site on the anode side of the PCM. As water is a product of the reaction, it is possible to supply a large portion of such water from reaction products at the cathode.
Unfortunately, known protonically-conductive membrane materials exhibit an undesirable characteristic. Specifically, water and some fuel and water molecules are carried from the anode to the cathode through the PCM in addition to the transferred protons. Fuel is primarily lost due to a phenomenon known as “methanol crossover,” which is caused by the fact that most currently available membranes allow fuel to pass through the PCM from the anode to the cathode and oxidized there without generating electricity. The lost fuel reduces the efficiency of the cell. In addition, as protons migrate through the membrane, they carry molecules of water through the PCM as well, a characteristic known as “water carryover.” The water that carries through the PCM may be far in excess of that generated by the recombination on the cathode. While this problem might be addressed by either return pumping of larger volumes of water, or excessive dilution of the stored fuel to maintain adequate dilution of the fuel at the anode, each of those approaches has clear disadvantages. What is needed is an improved membrane to limit water carryover especially, as well as methanol crossover fuel loss too, without equivalent reduction in total protonic exchange.